Conventionally, various kinds of document retrieval systems which retrieve documents in accordance with a retrieval request from a user have been developed. As a document retrieval system of this type, a document retrieval system which retrieves documents on a word basis by using a full text search technique or a morphological analysis technique is known. However, an enormous number of documents and an enormous amount of document data may satisfy one retrieval request. In such a case, the user needs to check the contents of each of the enormous number of retrieved documents on the display window and select a necessary document from them or extract necessary information. Recently, therefore, there has been developed a user interface which displays a list of summaries (or abstracts) of retrieved documents, in addition to the titles, creators, and the like of the respective retrieved documents, to save the labor of the user. In this case, as a summary, a sentence containing a keyword in a retrieved document, which is designated by the user, the first sentence of the retrieved document, or the like is generally used.
However, the determination as to whether the summary of each document displayed is necessary for a user is influenced by differences in viewpoint between the users or individual differences. The following document retrieval technique is therefore disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-255172 (reference 1). According to the document retrieval technique disclosed in reference 1, a sentence or information is extracted as an excerpt (excerpt data) from document data (original-document data) constituting each of many documents as retrieval targets. In this case, excerpt data is extracted in advance from the original-document data of each document stored in an original-document database on the basis of various viewpoints or references, which are called sentence patterns, for each sentence pattern. The excerpt data extracted for each sentence pattern are stored in a database (excerpt database) on a document basis. Of the excerpt data extracted from the document data of the retrieved documents, a list of excerpt data matching a sentence pattern selected by the user is displayed. According to such a document retrieval technique, excerpts (summaries) which are highly likely to be required by the user can be displayed.
Jpn. Pat. Appln. KOKAI Publication No. 2002-132811 (reference 2) discloses the following question answering document retrieval technique. According to the document retrieval technique disclosed in reference 2, a retrieval system (question answering document retrieval system) is provided with a retrieval request in question form from a user. Such a retrieval request in question form is a retrieval request in a natural language like, for example, “What is the price of XXX?” or “Who is the president of the YYY company?”, i.e., a question. In this case, a retrieval word set and a question classification are determined from a question. An associated document set is then retrieved from a document set on the basis of the retrieval word set and the question classification. Answers (words) to the question are extracted from the associated document set. Pairs of the extracted answers and documents (or the document numbers of the documents) containing the answers are presented, as answer results for the question, to the user by the retrieval system.